The invention relates to a nickel-base alloy including a continuous matrix composed of a solid solution of chromium in nickel and a precipitate granularly dispersed in and coherent with the matrix and composed of an intermetallic nickel compound. The invention also relates to an article of manufacture including a substrate formed of such a nickel base alloy.
Nickel-base alloys without a precipitate granularly dispersed in a nickel and chromium matrix and without having an inter-metallic nickel compound are widely used in different technical fields. U.S. Pat. No. 3,898,081 for example relates to nickel-base alloys, and, more particularly, to alloys especially useful as high precision resistor materials used in manufacturing resistors for various measurements circuits to control instrumentation. These nickel-base alloys include a combination of such additives as chromium, vanadium and gallium and have a resistivity of from 1.7 to 2.2 xcexcxcexa9xc3x97m. The content of gallium lies in the range of between 6 to 12%.
U.S. Pat. No. 3,907,555 relates to corrosion resistant precision casting alloys particularly suitable for use as dental alloy. The alloy is hot workable and hardenable and consists essentially, by weight of at least 60% nickel, 10 to 25% chromium, 1 to 7.5% gallium, 0.5 to 1.5% manganese and optionally tin, copper, silicon, aluminum, cobalt, and carbon to some percent.
The total amount of tin and gallium does not exceed 7.5%. In the alloy the same characteristics of gallium and tin are used to obtain good casting properties.
International Patent Application WO 82/03007 A1, corresponding to U.S. Pat. No. 4,459,262, relates to a cobalt and nickel alloy, in particular for the preparation of dental protheses. The alloy has sufficient qualities of corrosion and wear resistance, is cold-deformable and retains its color, is easily workable in a molten state and shows hardness values equivalent to those of noble metal alloys. Beside the base metals cobalt and nickel, the alloy contains as main components by weight 10 to 15% chromium and 0.2 to 4.5% gallium. The alloy can be used especially for preparing base plates, anchoring hooks and fastening hooks for mobile protheses.
A nickel-base alloy and an article of manufacture containing a substrate formed of such a nickel-base alloy is apparent from the book xe2x80x9cSuperalloys IIxe2x80x9d, edited by C. T. Sims, N. S. Stoloff and W. C. Hagel (editors), John Wiley and Sons, New York 1987. Of particular relevance in this context are chapter 4 xe2x80x9cNickel-base alloysxe2x80x9d, pages 97-134, chapter 7 xe2x80x9cDirectionally Solidified Superalloysxe2x80x9d, pages 189-214, and chapter 20 xe2x80x9cFuture of Superalloysxe2x80x9d, pages 549-562. The book discloses particular embodiments of such nickel-base alloys, termed as xe2x80x9csuperalloysxe2x80x9d. These superalloys are characterized by superior mechanical properties under heavy mechanical and thermal loads at temperatures amounting up to 90% of the respective melting temperatures.
A nickel-base superalloy can be characterized in general terms as set out above. In general, a nickel-base superalloy contains a continuous matrix composed of a solid solution of chromium in nickel and a precipitate granularly dispersed in and coherent with the matrix and composed of an intermetallic nickel compound. To specify the precipitate as coherent with the matrix means that crystalline structures of the matrix are continued into the grains of the precipitate. Thus, there are in general no physical boundaries between the matrix and the grains of the precipitate. Instead, an interface between the matrix and a grain of the precipitate will be characterized by a local change in chemical composition through a continuous, however strained, crystal lattice.
In a superalloy, both the matrix and the precipitate have a face-centered cubic crystal structure. The material of the matrix is usually specified as a xe2x80x9cgamma-phasexe2x80x9d, the material of the precipitate is specified as a xe2x80x9cgamma-prime-phasexe2x80x9d. The gamma-prime-phase has a composition that is generally specified as A3B, where A is generally nickel and B is generally aluminum or titanium. Generally, both the matrix and the precipitate are more or less highly alloyed; not all chromium is concentrated in the matrix, and not all aluminum and/or titanium is concentrated in the precipitate. Also, further elements are generally present in the alloy, and these elements are likewise distributed in the matrix as well as in the precipitate. Eventually, such elements may form other precipitates, particularly carbides or borides. Such compounds are formed with carbon or boron on one hand and elements like tungsten, molybdenum, hafnium, zirconium and others, as apparent from the book, on the other side. Carbides in particular play a more or less important role in commercially used superalloys. Boron is also frequently found in commercially used superalloys.
To manufacture a superalloy article with specified properties, not only control of its chemical composition is necessary, but also control of the manufacturing process which necessarily includes a heat treatment for the article after it has been brought to shape by casting or working. Normally, the heat treatment starts with a step called solutioning, where the superalloy is heated to a temperature near the incipient melting point to homogenize and dissolve precipitates which may have formed during casting or working. The solutioning will be finished by rapid cooling to retain the homogenous structure. Subsequently, at least one aging step will be performed by heating the article to a prescribed and carefully controlled temperature, in order to initiate the forming of the desired precipitate or the desired precipitates. Relevant particulars of such heat treatment processes may be found in the relevant chapters of the book.
Nickel-base superalloys to be used for the manufacture of gas turbine components like blades, vanes and heat shield elements are apparent from U.S. Pat. No. 5,401,307. The patent contains a survey of superalloys which are of concurrent practical importance, and the patent also elaborates on protective coatings which may be used to protect a superalloy article against corrosion and oxidation at high temperatures, as occurring during service in gas turbines.
Frequently a thermal barrier coating is used to extend the thermal loadability of a thus coated superalloy article to a higher temperature than without the thermal barrier layer. In general, a thermal barrier layer for a superalloy article is applied on a bond coating, which may be formed of an alloy or an intermetallic compound which itself has protective properties with respect to corrosion and erosion and is applied between the superalloy substrate and the ceramic thermal barrier coating. Examples of such protective coatings can be seen from U.S. Pat. No. 5,401,307 already mentioned.
U.S. Pat. No. 5,262,245 describes an effort to modify a superalloy in order to make it suitable to develop a thin film of aluminum on its surface, which film can be used to anchor a ceramic thermal barrier coating directly on the superalloy.
Recent efforts to improve creep rupture properties of nickel-base superalloys have resulted in alloys wherein the proportion of the intermetallic precipitate amounts up to 50% in parts by volume and even more. Therefore, these alloys have superior creep properties at temperatures above 750xc2x0 C. However, it has been observed that a steady increase of the proportion of the intermetallic precipitate in a superalloy leads to a remarkable embrittlement, since the pronounced brittleness of the intermetallic compounds that usually form the precipitate tends to dominate the mechanical properties of the superalloy. Finally, this results in an intolerable decrease in toughness. Furthermore, the solvability of chromium in the superalloy is remarkably reduced, since most of the chromium must be stored in the matrix, whose proportion must be reduced as the proportion of the precipitate is increased. This leads to a decrease in corrosion resistance, which as a rule is promoted by chromium. Corrosion resistance may not be a highly important property of a superalloy, since a protective coating is generally used in a high temperature application. However, a certain corrosion resistance must be retained even for the superalloy forming a substrate for a high temperature application, in order to avoid immediate failure of the substrate if the protective coating is lost by some kind of damage.
Additionally, long-time stability of the gamma-prime-phase of the precipitate at high temperatures may result in problems. By thermally activated diffusion processes, the precipitate may change its relevant properties. In particular, fine grains of the precipitate begin to grow within a process known as xe2x80x9cOstwald ripeningxe2x80x9d. Ostwald ripening also changes the shape of the grains of the precipitate from a basically cubic structure to a globular structure. Thereby, the grains lose their toughening properties at least partly, which can be verified by creep rupture tests at high temperatures.
It is accordingly an object of the invention to provide a nickel-base alloy and an article manufactured thereof that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which retains its potential for improvement of its creep rupture properties by increasing the proportion of precipitate and yet avoids the disadvantages of embrittling, Ostwald ripening and loss of solvability for chromium as explained.
With the foregoing and other objects in view there is provided, in accordance with the invention, a nickel-base alloy for high temperature application, including: a continuous matrix composed of a solid solution of chromium in nickel and a precipitate composed of an intermetallic nickel compound and granularly dispersed in and coherent with the continuous matrix, the intermetallic nickel compound containing gallium, the continuous matrix containing gallium oxide (Ga2O3) for providing a high corrosion resistance and having a balance of nickel, chromium and unavoidable impurities.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is described herein as embodied in a nickel-base alloy and an article manufactured thereof, it is nevertheless not intended to be limited to the details recited, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments.